Coupling of light from an array of laser transmitters to a matching array of optical waveguides is crucial for utilization of optical interconnects. Similarly critical is the coupling of light from the waveguide array to a matching array of photoreceivers.
In optical interconnects, each channel is built from a laser transmitter, a photodiode and a waveguide (typically optical fiber) connecting the two. Optical coupling between the elements is carried out using microlens that serve as a light focusing and collecting medium. Facilitation of dense optical interconnects is enabled using a large number of parallel optical channels. Thus, large, two-dimensional, matrices of laser diodes, microlenses and photodetectors are utilized. Given the large optoelectronic arrays, there are two major obstacles relevant to realization of a dense parallel optical interconnect. The first is coupling efficiency and the second optical crosstalk. These problems arise from the size and two-dimensional nature of the matrices; in a typical assembly scheme, the matrix of optical fibers is aligned directly above the optoelectronic matrix with a microlens array in between. Such an arrangement is prone to alignment errors resulting from the size of the matrices and the 6 degrees of freedom associated with each matrix. The key element controlling the degree of alignment errors is the microlens array design and there are several methods which have addressed the coupling efficiency and crosstalk issue of fiber light coupling with large optoelectronic matrices.
Typically, a bi-convex lens would be used for such a task as it would imply usage of a single lens with two optical surfaces. One surface would handle the light collection from either fiber or laser while the other surface focuses the light onto the fiber face or PD aperture. An example of such a lens is given in U.S. Pat. No. 8,090,230 and published US application number 2006/0098292. With such an approach, the alignment tolerances are limited because a single optical element is used.
A different approach, in which the bi-convex lens is converted into a 2-lens relay, is described in reference an article by F. Doany and others from IBM, entitled “160-Gb/s Bidirectional Parallel Optical Transceiver Module for Board-Level Interconnects Using a Single-Chip CMOS IC,” from the 2007 Electronic Components and Technology Conference. This fiber coupling method is based on assembly of several one-dimensional optoelectronic arrays (1×12 elements) which are assembled next to each other on top of the Silicon interposer. Thus, an inherent inaccuracy in the laser and photodiode pitch is introduced into the system thereby limiting the size of the microlens array due to accumulation of alignment errors.
The crosstalk problem for large matrices was addressed with a 2-lens relay as described in U.S. Pat. No. 5,857,042. In this approach, the laser sources and first lenses are offset relative to each other. An identical but opposite offset is introduced between the second lens array and photodetector array as well. Any stray light will be focused away from neighboring channels thereby minimizing crosstalk. Such accurate offsets are difficult to achieve in a manufacturing environment making this solution impractical.